This invention relates generally to microwave power combiners and power dividers, and more particularly, to microwave combiners and dividers of radial configuration. The term "microwave" is generally applied to electro-magnetic signals and devices operating in the frequency range from 300 MHz (megahertz) to 300 GHz (gigahertz). To obtain high powers at high frequencies, the outputs of multiple oscillator or amplifier devices must be combined. There is, therefore, a need for microwave power combiner operable over a wide band of frequencies and capable of handling high powers. Other applications, such as phased-array antennas, require a power dividing function, in which a single high-power radio-frequency (rf) input signal is to be split into a number of output signals, usually of equal but smaller powers.
Various configurations have been proposed to provide the power combining or dividing function, including Kurokawa-type combiners, magic-tee hybrid couplers and microstrip power dividers or combiners. The Kurokawa device is basically a cavity to which is coupled a number of coaxial waveguides providing separate power inputs, such as from IMPATT diodes (employing impact-ionization avalanche transit-time properties). Although power combiner devices of this type are satisfactory for some applications, their chief limitation is a relatively narrow bandwidth, arising from their resonant nature. Magic tee or hybrid couplers have good bandwidth characteristics but are usually limited to four or eight input sources. Moreover, they have high losses at millimeter-wave frequencies (above 30 GHz). Similarly, microstrip combiners or dividers have high losses at high frequencies and are, therefore, incapable of handling high powers at these frequencies.
Radial line combiners using microstrip structuress have been disclosed in U.S. Pat. Nos. 4,371,845 to Pitzalis, Jr. 4,234,854 to Cohn et al., and 4,032,865 to Harp et al. Other attempts to produce a wideband non-resonant power combiner structure include a so-called radial line combiner disclosed in U.S. Pat. No. 3,582,813 to Hines, in which solid-state power-generating devices are disposed around a central coaxial output line, to which they are coupled. Another proposed solution to the problem is the conical power combiner disclosed in U.S. Pat. No. 4,188,590 to Harp et al. In a paper entitled "A 6-GHz GaAs FET Amplifier with TM-Mode Cavity Power Combiner," by Naofumi Okubo et al., 1983 IEEE MTT-S Digest, pp. 276-77, an improved frequency response is obtained by employing two radial cavities coupled together in a series stack, in an axial sense.
In all radial wave combiners or dividers, having a central port and multiple peripheral ports, a desired performance response is typically obtained by first loading the peripheral ports with a lossy material and matching the central port to conform with the characteristics of the radial waveguide. Then peripheral port matching is attempted, but the resulting complex impedence presented at the central port restricts the operating bandwidth of the device, and limits its performance.
In essence, the only prior-art approach to achieving a desired frequency response in a power combiner divider is largely an empirical one. In brief, the physical parameters of the device are modified until the desired characteristic is approached. Designing a combiner or divider with a broadband frequency response is particularly difficult and has long provided a challenge to designers of microwave devices.
It will be appreciated from the foregoing that there is a need for a more reliable approach to the design of radial microwave dividers or combiners. The present invention is directed to this end.